Why pay for another computing device when the one in your hand can accomplish everything you need?
Personal device and work device.
Backup device in case one is broken / unavailable.
Different types and amounts of storage for different purposes - conventional disk is still a lot cheaper than SSD. Different hardware interfaces, or differently-accessible ones. (My phone has a MicroSD "slot", but I have to take the phone apart to get to it. It's great for additional storage, not great if I want to read the SD card from my camera. I could carry around a USB SD reader, but then we're back in the game of carrying a bunch of hardware around.) Bigger batteries in the laptop form factor.
If I'm going to use a phone in "laptop mode", I'll be using a full-size keyboard and screen. That's a laptop; it just has a detachable CPU with its own little screen, packaged as a phone. I don't see any particular advantage to that configuration.
Today I have a work laptop, a personal laptop, and a phone. I don't want to replace any of those with one of the others. I like separation of concerns. That means I often carry three devices around, but apparently either I have superhuman strength, or a lot of people like to whine about what is really not much weight. (I suspect a little exercise might do them good.)
Of course, I'm under no illusions that my preferences match those of any significant number of consumers. But every time I read one of these posts about how obviously everyone will be doing X in a few years, I really have to wonder whether the author has ever actually met other people. There are a bunch of us, and we don't all hold the same opinions.